Purified carboxylic acids, and particularly aromatic dicarboxylic acids are industrially important. For example purified terephthalic acid (TPA) is a starting material for the formation of polyester resin, which is, in turn, used to make many materials of commerce having a variety of utilities. Purified terephthalic acid is formed from “crude” terephthalic acid conventionally by a number of purification methods, often with the aid of catalysts. Many of the currently available methods for purifying crude terephthalic acid are not completely satisfactory either from an engineering or an economic standpoint. Yet the purity of terephthalic acid is an important determinant in the formation of polyester resin.
Many of the problems of existing and prior systems for producing purified terephthalic acid stem from the difficulties in economically running reaction systems to produce good yields of crude terephthalic acid, compounded by the difficulties of refining crude terephthalic acid to eliminate impurities and unwanted components to produce purified terephthalic acid of a quality suitable as a starting material for producing polyester. Concomitant problems in prior systems include the high capital investment required for PTA plants, the severity of operating conditions of prior processes, both for the production of crude terephthalic acid, and for its purification, and the need for handling catalyst systems and reaction solvents, as well as reaction byproducts in a way such that environmental problems are minimized, and loss of material is also controlled.
One important factor in the production of purified terephthalic acid is the formation of crystals having a size and shape that provide them with good handling characteristics, washability, and filterability in the PTA manufacturing process, and also provide easier handling and better processability in a polyester process.
U.S. Pat. No. 2,949,483, by Ham et al., describes dissolving terephthalic acid in N-methyl-2-pyrrolidinone then precipitating a “salt complex” containing terephthalic acid and N-methyl-2-pyrrolidinone. The solid is washed with water to remove the N-methyl-2-pyrrolidinone and yield purified TPA. The patent claimed a recovery rate of about 60% to 95% of the weight of the crude terephthalic acid.
U.S. Pat. No. 5,840,968, by Lee, et al., describes a method and apparatus for purifying crude terephthalic acid from a liquid dispersion also containing impurities of unreacted starting materials, solvents, products of side reactions and/or other undesired materials. The method uses the steps of filtering the dispersion to form a crude terephthalic acid filter cake, dissolving the filter cake in a selective crystallization solvent at an elevated temperature to form a solution, crystallizing purified terephthalic acid from the solution in the crystallization solvent by reducing the pressure and temperature of the solution, and separating the crystallized purified terephthalic acid from the solution. The selective crystallization solvent is non-aqueous, non-corrosive and essentially non-reactive with terephthalic acid. Examples of selective crystallization solvents described are N-methyl pyrrolidone or dimethyl acetamide.
U.S. Pat. No. 5,929,274, by Lamshing, et al., describes a method for reducing carboxybenzaldehyde isomers in crude terephthalic and/or isophthalic acids (IPA). Crude TPA or IPA is dissolved in N-methyl pyrrolidone and is subsequently contacted with an oxidant, such as substantially anhydrous hydrogen peroxide, to convert the carboxybenzaldehyde isomer (4-CBA or 3-CBA) to TPA or IPA under moderate temperature and pressure conditions.
U.S. Pat. No. 6,054,610, by Lee et al., describes a method and apparatus for preparing purified terephthalic acid and optionally isophthalic acid from mixed xylenes. The method purifies the oxidation reactor effluent containing a mixture of terephthalic acid and isophthalic acid as well as minor amounts of 4-carboxybenzaldehyde (4-CBA), 3-carboxybenzaldehyde (3-CBA), and toluic acid isomers, to produce purified terephthalic acid and, optionally, purified isophthalic acid in an integrated process.
U.S. Pat. No. 6,140,534, by Lee et al., describes a method for preparing isophthalic acid from metaxylene and especially for purifying crude isophthalic acid (IPA) produced in the course of such method, or otherwise, from a liquid dispersion thereof also containing unreacted starting materials, solvents, products of side reactions and/or other undesired materials. The purifying portion of the method includes the steps of: (1) filtering the dispersion to form a crude IPA filter cake; (2) dissolving the filter cake in a selective crystallization solvent at an elevated temperature to form a solution; (3) crystallizing purified IPA from the solution in the crystallization solvent by reducing the temperature, or pressure, or both of the solution; (4) separating the crystallized purified IPA from the solution; and (5) re-dissolving or soaking the washed purified IPA cake at elevated temperature, to remove the final traces of the crystallization solvent and obtain the desirable particle sizes and shape. The selective crystallization solvent is typically N-methyl pyrrolidone.